This theme do not have any recommended size it works as a background image cover. However you can modify the post height as of your featured image. Just go to Appearance > Customize > Layout > Post Layout there you can see the Post Grid Height option to modify the height of the posts.
As we already said that this theme uses background image cover for post grid view you can use any size. However, if you need a pixel perfect view you have to set the image min 360px × 252px.
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